In recent years, GaN compound semiconductor materials have been attracting attention as semiconductor materials for short-wavelength light-emitting devices. GaN compound semiconductor materials are formed on substrates such as various oxide substrates and III-V compounds, including a sapphire single crystal, by a metal-organic chemical vapor deposition (MOCVD) method, a molecular beam epitaxy method (MBE method), and the like.
Incidentally, external quantum efficiency of a light-emitting device is represented as a value in which its light extraction efficiency and internal quantum efficiency are multiplied. The term “internal quantum efficiency” is a percentage of energy to be converted into light in the energy of electric current injected into the device. The term “light extraction efficiency” is a percentage of light which can be extracted to the outside in the light generated inside a semiconductor crystal.
It has been said that the internal quantum efficiencies of light-emitting devices have been improved to approximately 70 to 80% due to an improvement in crystal conditions and a study for the structures, and sufficient effects on the amount of injected current have been obtained.
However, light extraction efficiencies with respect to injected current are low in general in not only the GaN compound semiconductor materials, but also light-emitting diodes (LEDs), it is difficult to say that internal light emission with respect to injected current is sufficiently extracted to the outside.
The reason that light extraction efficiencies are low is that light emitted from light-emitting layers are repeatedly reflected and absorbed depending on crystal materials in the LED structures, and the probability that the light cannot be extracted to the outside is high in reflection at a critical angle or more due to Snell's law.
In order to improve light extraction efficiency, technology in which light extraction efficiency is improved such that a light extracting surface is roughened, and various angles are provided to the light extracting surface, which improves the light extraction efficiency has been proposed (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2003-218383 and Japanese Unexamined Patent Publication (Kokai) No. 2005-64113).
However, by the roughening of the surface manufactured by the proposed technology, the roughened pattern is formed from plane surfaces or into a pyramid structure, and the roughened area is not entirely impregnated with a resin material depending on its viscosity when the device is formed as an LED package, a light extraction effect due to the roughening or a light extraction effect according to a resin refractive index to be expected is not sufficiently obtained, which leads to the problem that the process is made cumbersome and complicated because it is necessary to change a pressure after resin infusion in order to make the resin thoroughly adhered to the roughened surface.
Further, with respect to a method for performing roughening, because a method for roughened mask patterning or the like is required, it is necessary to follow a cumbersome procedure in the process, which is the problem in manufacturing.
On the other hand, as a characteristic of GaN compound semiconductor materials, lateral current diffusion is little. Therefore, an electric current is injected into only a semiconductor immediately beneath an electrode, and light emitted from a light-emitting layer is blocked by the electrode, and is not extracted to the outside. Usually, in this type of light-emitting device, a transparent electrode is used as a positive electrode, and light is extracted through the transparent electrode.
A conventional transparent electrode has a layered structure in which oxide such as Ni or Co, and Au or the like serving as contact metal are combined. In recent years, a layered structure whose transparency is improved by making a film thickness of contact metal as thin as possible by using oxide such as ITO having higher conductivity is employed as a positive electrode, and light from a light-emitting layer is efficiently extracted to the outside.
Further, there has been proposed a structure in which a pattern of a positive electrode is made into a pattern having openings, for example, a lattice pattern, and emission of light is extracted from the openings (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2004-179491). However, in this method, portions from which emission of light is extracted are limited to the portions around the positive electrode, and emission of light is not extracted from the central portions of the openings, and in the present circumstances, satisfactory light extraction efficiencies have not been increased.